Let $f = x+\sum_{n=2}^{\infty} a_n x^n$ and $g = x+\sum_{k=2}^{\infty} b_n x^n$ be two formal series without constant term. Then $$ f \circ g(x) =x+ \sum_{k=2}^{\infty} b_k x^k+\sum_{n=2}^{\infty} a_n (x+\sum_{k=2}^{\infty} b_k x^k)^n, $$ Similarly, $$ g \circ f(x) =x+ \sum_{n=2}^{\infty} a_n x^n+\sum_{k=2}^{\infty} b_k (x+\sum_{n=2}^{\infty} a_n x^n)^k. $$ Now, we have $$ \left(\sum_{k=1}^{\infty} b_k x^k \right)^n = \sum_{i_1 + \dotsm + i_k = n} b_{i_1} \dotsm b_{i_k} x^n, \ i_k \geq 1. $$ I want to get the conditions when $f(g(x))=g(f(x))$ i.e., when the two power series commutes each other. Then, $$x+ \sum_{k=2}^{\infty} b_k x^k+\sum_{n=2}^{\infty} a_n (x+\sum_{k=2}^{\infty} b_k x^k)^n=x+ \sum_{n=2}^{\infty} a_n x^n+\sum_{k=2}^{\infty} b_k (x+\sum_{n=2}^{\infty} a_n x^n)^k,$$ i.e., $$\sum_{k=2}^{\infty} b_k x^k+\sum_{n=2}^{\infty} a_n (x+\sum_{k=2}^{\infty} b_k x^k)^n=\sum_{n=2}^{\infty} a_n x^n+\sum_{k=2}^{\infty} b_k (x+\sum_{n=2}^{\infty} a_n x^n)^k$$, i.e, $$\sum_{k=2}^{\infty} b_k x^k+\sum_{n=2}^{\infty} a_n x^n+\cdots+\color{blue}{\sum_{n=2}^{\infty} a_n (\sum_{k=2}^{\infty}b_kx^k)^n}=\sum_{n=2}^{\infty} a_n x^n+\sum_{k=2}^{\infty} b_k x^k+\cdots+\color{blue}{\sum_{k=2}^{\infty} b_k(\sum_{n=2}^{\infty}a_nx^n)^k}. $$ The first two terms of both sides cancels out and comparing the last term of both sides i.e, coefficients of $x^{kn}$, we get $$ a_n \left(\sum_{i_1 + \dotsm + i_k = n} b_{i_1} \dotsm b_{i_k}\right) = b_n \left(\sum_{i_1 + \dotsm + i_k = n} a_{i_1} \dotsm a_{i_k}\right),\ i_k \geq 2. $$ But how to deal with middle $\cdots$ terms ??
That is, what is criteria so that $f(g(x))=g(f(x))$. ?
No idea why you got stuck.
$$\sum_{n\ge 1} a_n (\sum_{m\ge 1} b_m x^m)^n=\sum_{n\ge 1} a_n \sum_{r \in \Bbb{Z}_{\ge 1}^n} \prod_{j=1}^n b_{r_j} x^{r_j} = \sum_{l\ge 1} x^l \sum_{n=1}^l a_n \sum_{r \in \Bbb{Z}_{\ge 1}^n, \sum_{j=1}^n r_j = l}\prod_{j=1}^n b_{r_j} $$ and $\sum_{n\ge 1} a_n (\sum_{m\ge 1} b_m x^m)^n=\sum_{n\ge 1} b_n (\sum_{m\ge 1} a_m x^m)^n$ iff $$\sum_{n=1}^l a_n \sum_{r \in \Bbb{Z}_{\ge 1}^n, \sum_{j=1}^n r_j = l}\prod_{j=1}^n b_{r_j}=\sum_{n=1}^l b_n \sum_{r \in \Bbb{Z}_{\ge 1}^n, \sum_{j=1}^n r_j = l}\prod_{j=1}^n a_{r_j}$$